Are there any guidelines for subpanel mounting that one should take note of? It will be in the basement, and there is no more room to mount the subpanel on the same backing that the main is attached to.

1. Height and distance from windows.
2. Substrate to be mounted to (I have seen blackened or dark wood, not sure if the dark wood is specially treated or simply painted black).

1) No. Only normal clearances. 30"wX36"deepX78"h
2) No rule with regard to the wood. Either use wood pr spacers to give an air gap. Obviously wood is the better choice.
I would mount another piece of wood the same thickness as the main is on right next to the existing.